What is the Opening Size (or Viewing Area) on a Fireplace or Fire Box?

The Opening Size (or Viewing Area) of a fireplace or fire box refers to the dimensions of the area where the flames are visible. It does not include the outer frame, flange, or any external measurements—just the innermost box where the fire is contained.

In the image you provided, the firebox has an Opening Size (Viewing Area) of 68.1 inches. This is the space where you would see the flames when the firebox is in use.